The present invention relates of production of heavy hydrocarbons contained in an underground formation by an assisted recovery process using wells drilled with an essentially horizontal section, said wells having an initial practically vertical section starting at the surface of the ground, followed by an inclined or horizontal section extending into the formation.
Extraction of heavy hydrocarbons from an underground formation implies production mechanisms designed essentially to reduce viscosity and to cause displacement followed by aspiration of the heavy oil into wells, and finally bringing it to the surface. There are two methods for generating the energy required for displacement: producing this energy at the surface, in the case of injection of hot fluids (U.S. Pat. No. 4,325,432) or the creation of such fluids in the formation, as in the case of in situ combustion (U.S. Pat. No. 4,501,326).
This principle of reducing the viscosity of heavy oil by heating is generally accompanied by careful selection of well-drilling locations to use the injected energy with increased efficiency.
Hence, development in recent years has stressed the use of wells drilled horizontally in a layer of a formation to increase production yield. Horizontal wells have made it possible (a) to reach reservoirs of hydrocarbons in locations sometimes inaccessible to vertical wells, and (b) have shown improved profitability in production and extraction of petroleum located in certain types of formations.
Hence, initial developments were directed at using horizontal wells to produce heavy hydrocarbons by injecting steam.
Steam injected into a will diffuses in the hydrocarbon, reducing its viscosity and starting its displacement toward a producing well by thermal transmission.
A method of this kind is described in U.S. Pat. No. 4,700,779, in which the reservoir containing the heavy hydrocarbon is pierced by a series of wells with horizontal drains whose horizontal sections are parallel to one another and extend longitudinally in the reservoir.
The hydrocarbon production process is worked by activating a first well in a first stage and a second well located at one end of the formation by injecting steam, and capturing the hydrocarbon in a second step after starting heating in the second well immediately adjacent, which is then transformed from an injector into a producer.
When the opening formed by the steam reaches the second well, steam injection is suspended in the first well and replaced by water injection to maintain sufficient pressure in the reservoir.
It is then sufficient to shift the functions of the wells to extract, at a third well, the hydrocarbon set in motion by the injection of steam at the second well.
Such a technique has been used successfully for production from a hydrocarbon reservoir, but this system can be used only when the hydrocarbon is contained in a single reservoir, while in many cases it is divided among reservoirs superimposed on one another and separated by impermeable secondary rocks. In such cases, each reservoir must be treated individually by the process described above, which rapidly leads to complications in controlling the wells (transmission of commands to switch from production to injection and vice versa) when the number of stratified layers is large.